1. Field of the Invention
The present invention relates to an oscillator used in a high frequency band such as the millimeter wave band, etc., and more particularly to a high-frequency oscillator in which the construction of the area where an oscillation element and a resonator are coupled is improved.
2. Description of the Related Art
An oscillator to be used in a high frequency band such as the millimeter wave band, etc., has been disclosed in Japanese Unexamined Patent Application Publication No. 2001-102871 (Reference No. 1). In that document, an oscillation circuit is constructed by providing a microstrip line and a Gunn diode on a dielectric substrate. Furthermore, a dielectric microstrip line is disposed between upper and lower conductor plates to construct an NRD guide (nonradiative dielectric waveguide) as an output transmission line. The above microstrip line in the oscillation circuit and the NRD guide are coupled.
In that oscillator, the cut-off frequency of the NRD guide is set so that the fundamental wave in an oscillation signal in the oscillation circuit is cut off and the harmonics are propagated. Accordingly, in an oscillator making use of the harmonics, the fundamental wave can be sufficiently suppressed and low loss can be realized.
On the other hand, in the Japanese Unexamined Patent Application Publication No. 7-212131 (Reference No. 2), a millimeter wave oscillator shown in FIGS. 10 and 11 is disclosed. In that oscillator, a case is composed of a casing main body 101 and a lid 102, and a dielectric substrate 103 and a Gunn diode 104, serving as an oscillation element, are disposed inside the casing main body 101. The Gunn diode 104 is coupled to a metal microstrip line formed on the substrate 103. The other end, opposite to the side to which the Gunn diode is coupled, of the metal microstrip line 105, is connected to a high frequency connector 106.
In these millimeter wave oscillators described above, the inside height and width of a section, perpendicular to the direction of transmission of an oscillation output, of the case are set to be one half of the wavelength of the oscillation or smaller, and accordingly the oscillator can be made smaller and the productivity can be increased.
In the oscillator described in the above Reference No. 1, although the fundamental wave as an unwanted mode can be suppressed, leakage of the unwanted mode cannot be sufficiently prevented only by setting the cut-off frequency of the NRD guide as described above, and accordingly the oscillation output cannot be obtained in an efficient and stable way.
On the other hand, in the millimeter wave oscillator described in the above Reference No. 2, the inside dimensions of a section, perpendicular to the direction of transmission of an oscillation output, of the case are set to be one half of the wavelength of the oscillation or smaller. Accordingly, for example, when the wavelength of the oscillation becomes on the order of 76 to 77 GHz, the inside dimensions of the case become as small as 1.95 to 1.97 mm. That is, it is inevitable that the height and width of the case have such small dimensions. Therefore, there is a problem in that it becomes difficult to house the resonator coupled to the oscillation element and other circuit components inside the case.